Builds often involve one tool generating a file that is consumed later by another tool.

In some cases, the intermediate file is source code to be compiled. (For example, SWIG, lex, and yacc all do this.) In those cases, users sometimes have a desire to modify the generated source before it is compiled. The need to do this most often comes up when developing something new and wrestling with problems in the code generation process.

Vesta makes this more difficult than make-style build systems. Every build must be a completely specified recipe for going from initial sources to final results. So how could a developer who needs to modify an intermediate file do so under Vesta?